In the rest of this exercise, we'll develop a command for generating a heart
rate signal, add it to the WAVE menu, and test it. First, set the Start and End fields to 0 and 1:0 respectively, and click on
to regenerate the annotation file. Open
the annotation file using the Load window.

Generating a heart rate signal is simple enough, given a beat annotation file
and
tach,
a program supplied as part of the WFDB Software Package. tach reads an annotation file, calculates an instantaneous heart rate for each
cardiac cycle from the reciprocal of the interval between successive beat
annotations, and then uniformily resamples this signal (by default, at 2 Hz).
Referring to the
WFDB Applications Guide,
we find the command we need to generate a heart rate signal from
`100s.qrs':

tach -r 100s -a qrs -f 0 -t 1:0

If this command is typed into the Analysis Commands window, it prints:

73.9726
73.9726
73.7238
.
.
.
75.7788
74.5703
73.8015

These numbers are the calculated samples of the heart rate signal (the units
are beats per minute).

We can generalize this command as:

tach -r $RECORD -a $ANNOTATOR -f $START -t $END

where the parameters $RECORD, $ANNOTATOR, etc., are replaced by
the appropriate values when the command is executed. This is the form of the
command we should add to WAVE 's menu; by parameterizing the arguments
in this way, we can use WAVE to select a record, annotator, and segment
to be analyzed as for the other commands in the menu. To add this command
to the menu, click on
again. Position the cursor at the end
of the menu file and add the line:

Heart rate<TAB>tach -r $RECORD -a $ANNOTATOR -f $START -t $END

where <TAB> represents one or more TAB characters; at least one
TAB must be present (spaces don't count). Be sure to end the line with
RETURN. Save the menu (choose Save from the textedit window's
menu), and quit from textedit. Now
click on
in the Analyze window. The Analyze
window will disappear briefly, then reappear (possibly in a different location,
depending on your window manager) with a new
button. If
you click on this button, WAVE executes the tach command as above.

You might prefer to look at heart rate as a signal rather than as a column of
numbers. tach has the capability of generating an output signal file
instead of text. To try this out, edit the menu file again, and add `-o
hr_$RECORD' to the end of the entry. (You may split the command across two
lines using a `' at the end of the first line if you wish.)
Once again, save the menu file, quit from textedit, click on
, and on
. This time, there is no text
output from tach; if you enter `hr_100s' in the Record
field of the Load window, however, WAVE displays the calculated
heart rate signal.

Figure 3.2:
Heart rate signal.

(You may wish to change the time and amplitude scales using the View
window, in order to see more of the signal at once. If you set the
time scale to 250 mm/min and the amplitude scale to 100 mm/mV, you
will be able to see the heart rate signal for the entire record as in
figure 3.2; the
periodic modulation is due to respiratory sinus arrhythmia.)